Precoding signaling in a mimo wireless communication system

ABSTRACT

A method for performing data transmission between a transmitter and a receiver. The method includes the steps of generating a feedback message at the receiver in response to data received from the transmitter, assigning an identifier for the feedback message, storing the feedback message in association with the identifier in the receiver, transmitting the feedback message and the identifier to the transmitter, determining, at the transmitter, transmission format for data to be transmitted to the receiver based on the feedback message received from the receiver; and transmitting data and a control message, by the transmitter, using the determined transmission format, with the control message comprising the identifier of the feedback message based on which the transmission format is determined.

PRIORITY

This application makes reference to, claims all benefits inuring under35 U.S.C. §119 and 120 from, and incorporates herein a provisionalapplication filed in the U.S. Patent & Trademark Office on the 5 of Feb.2007 and there duly assigned Ser. No. 60/899,578.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for transmitting data in aclosed-loop multiple input multiple output system, and morespecifically, a method for transmitting information regardingtransmission format in a closed-loop multiple input multiple outputsystem.

2. Description of the Related Art

Orthogonal Frequency Division Multiplexing (OFDM) is a popular wirelesscommunication technology to multiplex data in frequency domain.

A multiple antenna communication system, which is often referred to asmultiple input multiple output (MIMO) system, is widely used incombination with OFDM technology, in a wireless communication system toimprove system performance.

In a MIMO system, both transmitter and receiver are equipped withmultiple antennas. Therefore, the transmitter is capable of transmittingindependent data streams simultaneously in the same frequency band.Unlike traditional means of increasing throughput (i.e., the amount ofdata transmitted per time unit) by increasing bandwidth or increasingoverall transmit power, MIMO technology increases the spectralefficiency of a wireless communication system by exploiting theadditional dimension of freedom in the space domain due to multipleantennas. Therefore MIMO technology can significantly increase thethroughput and range of the system.

When the transmission channels between the transmitters and thereceivers are relatively constant, it is possible to use a closed-loopMIMO scheme to further improve system performance. In a closed-loop MIMOsystem, the receiver informs the transmitter of feedback informationregarding the channel condition. The transmitter utilizes this feedbackinformation, together with other considerations such as schedulingpriority, data and resource availability, to optimize the transmissionscheme.

A popular closed-loop MIMO scheme is MIMO precoding. With precoding, thedata streams to be transmitted are precoded, i.e., pre-multiplied by aprecoding matrix, before being passed on to the multiple transmitantennas in a transmitter.

In a contemporary closed-loop MIMO precoding scheme, when a transmitterprecodes data before transmitting the data to a receiver, thetransmitter informs the receiver of the precoding information such as anidentification of the precoding matrix by transmitting dedicated pilots(also referred to as reference signals) or explicit control informationthat carries the precoding information. A significant problem with thisapproach is that the control information inefficiently consumes asignificant amount of system resources and degrades the overall systemthroughput and capacity.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved system and an improved method for transmitting data in aclosed-loop multiple input multiple output (MIMO) system.

It is another object to provide an improved system and an improvedmethod that is capable of transmitting data in a closed-loop MIMO systemto save system resources and improve overall system throughput andcapacity.

According to one aspect of the present invention, there is provided asystem and a method for performing data transmission between atransmitter and a receiver, by generating a feedback message at thereceiver in response to a reference signals or a pilot signal receivedfrom the transmitter, assigning an identifier for the feedback message,storing the feedback message in association with the identifier in thereceiver, transmitting the feedback message and the identifier to thetransmitter, determining, at the transmitter, the transmission formatfor data to be transmitted to the receiver based on the feedback messagereceived from the receiver; and transmitting data and a control message,via the transmitter, using the determined transmission format, with thecontrol message comprising the identifier of the feedback message basedon which the transmission format is determined.

When the receiver receives data and the identifier of the feedbackmessage from the transmitter, the receiver may look up informationcorresponding to the transmitted identifier and process the receiveddata according to that information.

When the feedback message transmitted from the receiver and received bythe transmitter contains errors, the transmitter may determinetransmission format based on another feedback message received by thetransmitter prior to the erroneous feedback message.

The identifier of the feedback message may be a number.

The number may indicate the index of the feedback message in a series offeedback messages transmitted by the receiver, with the smallest numberindicating that the feedback message being the first one of the seriesof feedback message transmitted by the receiver.

Alternatively, the number may indicate the index of the feedback messagein a series of feedback messages previously received by the transmitter,with the smallest number indicating that the feedback message being themost recent feedback message that is received by the transmitter.

Still alternatively, the number may a subframe number during which thefeedback message is transmitted.

The number may be represented by binary digits.

The transmission format may be established in dependence upon frequencysubbands in which the data is to be transmitted.

According to another aspect of the present invention, there is provideda system and a method for performing data transmission between atransmitter and a receiver, by generating a feedback message at thereceiver in response to a reference signal or a pilot signal receivedfrom the transmitter, transmitting the feedback message to thetransmitter, storing information in the feedback message in thereceiver, deciding, at the transmitter, whether to transmit data to thereceiver according to a first transmission format determined based onthe feedback message received from the receiver, or according to asecond transmission format which is not related to the feedback messagereceived from the receiver, and transmitting data and a control message,by the transmitter, using either the first transmission format or thesecond transmission format, and when the first transmission format isused, informing the receiver that the transmission format is determinedbased on the feedback message received from the receiver, and when thesecond transmission format is used, informing the receiver that thetransmission format is not related to the feedback message received fromthe receiver and transmitting the second transmission format to thereceiver.

When the first transmission format is used, the transmitter may informthe receiver that the transmission format is determined based on thefeedback message received from the receiver by including a bit ‘0’ inthe control message transmitted to the receiver.

When the second transmission format is used, the transmitter may informthe receiver that the transmission format is not related to the feedbackmessage received from the receiver by including a bit ‘1’ in the controlmessage transmitted to the receiver.

According to still another aspect of the present invention, there isprovided a system and a method for performing data transmission betweena transmitter and a receiver, including the steps of generating afeedback message at the receiver in response to a reference signal or apilot signal received from the transmitter, transmitting the feedbackmessage to the transmitter, transmitting data from the transmitter tothe receiver, transmitting a control message which carries atransmission format used for the data transmission, decoding, at thereceiver, the control message in order to obtain the transmission formatof the data transmitted from the transmitter, and, when the decoding issuccessful, processing the data received from the transmitter accordingto the obtained transmission format, and when the decoding isunsuccessful, processing the data received from the transmitteraccording to the most recent feedback message that the receiver has sentto the transmitter.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of this invention, and many of theattendant advantages thereof, will be readily apparent as the samebecomes better understood by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawings in which like reference symbols indicate the same or similarcomponents, wherein:

FIG. 1 is an illustration of an Orthogonal Frequency DivisionMultiplexing (OFDM) transceiver chain suitable for the practice of theprinciples of the present invention;

FIG. 2 is an illustration of a multiple input multiple output (MIMO)system suitable for the practice of the principles of the presentinvention;

FIG. 3 is an illustration of a single-code word MIMO scheme suitable forthe practice of the principles of the present invention;

FIG. 4 is an illustration of a multi-code word MIMO scheme suitable forthe practice of the principles of the present invention;

FIG. 5A and FIG. 5B are examples of precoding in a precoding MIMO-systemsuitable for the practice of the principles of the present invention;

FIG. 6 is an illustration of an example of MIMO precoding on differentsubbands suitable for the practice of the principles of the presentinvention;

FIG. 7 is an illustration of an example of MIMO rank on differentsubbands suitable for the practice of the principles of the presentinvention;

FIG. 8 is an illustration of an example of MIMO layer ordering ondifferent subbands for a 2×2 MIMO system suitable for the practice ofthe principles of the present invention;

FIG. 9 is an illustration of control signaling in a wirelesscommunication system suitable for the practice of the principles of thepresent invention;

FIG. 10 illustrates MIMO feedback and signaling according to a firstembodiment of the principles of the present invention;

FIG. 11 illustrates MIMO feedback and signaling according to a secondembodiment of the principles of the present invention;

FIG. 12 illustrates MIMO feedback and signaling according to a thirdembodiment of the principles of the present invention;

FIG. 13 illustrates MIMO feedback and signaling according to a fourthembodiment of the principles of the present invention;

FIG. 14 illustrates MIMO feedback and signaling according to a fifthembodiment of the principles of the present invention;

FIG. 15 illustrates MIMO feedback and signaling according to a sixthembodiment of the principles of the present invention;

FIG. 16 illustrates MIMO feedback and signaling according to a seventhembodiment of the principles of the present invention;

FIG. 17 is a flow chart showing the processing of MIMO signals accordingto an eighth embodiment of the principles of the present invention; and

FIG. 18 illustrates an example of Physical Downlink Control Channel(PDCCH) containing MIMO transmission format information according to theprinciples of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. As those skilled in the art would realize,the described embodiments may be modified in various different ways, allwithout departing from the spirit or scope of the present invention.

FIG. 1 illustrates an Orthogonal Frequency Division Multiplexing (OFDM)transceiver chain. In a communication system using OFDM technology, attransmitter chain 110, control signals or data 111 is modulated bymodulator 112 and is serial-to-parallel converted by Serial/Parallel(S/P) converter 113. Inverse Fast Fourier Transform (IFFT) unit 114 isused to transfer the signal from frequency domain to time domain. Cyclicprefix (CP) or zero prefix (ZP) is added to each OFDM symbol by CPinsertion unit 116 to avoid or mitigate the impact due to multipathfading. Consequently, the signal is transmitted by transmitter (Tx)front end processing unit 117, such as an antenna (not shown), oralternatively, by fixed wire or cable. At receiver chain 120, assumingperfect time and frequency synchronization are achieved, the signalreceived by receiver (Rx) front end processing unit 121 is processed byCP removal unit 122. Fast Fourier Transform (FFT) unit 124 transfers thereceived signal from time domain to frequency domain for furtherprocessing.

The total bandwidth in an OFDM system is divided into narrowbandfrequency units called subcarriers. The number of subcarriers is equalto the FFT/IFFT size N used in the system. In general, the number ofsubcarriers used for data is less than N because some subcarriers at theedge of the frequency spectrum are reserved as guard subcarriers. Ingeneral, no information is transmitted on guard subcarriers.

FIG. 2 illustrates a multiple input multiple output (MIMO) system. In aMIMO system, transmitter 130 and receiver 140 are respectively equippedwith multiple antennas 135 and 145. Therefore, transmitter 130 iscapable of transmitting independent data streams 131 simultaneously inthe same frequency band.

An example of a single-code word MIMO scheme is given in FIG. 3. In caseof single-code word MIMO transmission, a cyclic redundancy check (CRC)152 is added to a single data stream 151 and then coding 153 andmodulation 154 are sequentially performed. The coded and modulatedsymbols are then demultiplexed 155 for transmission over multipleantennas 156.

In case of multiple-code word MIMO transmission, shown in FIG. 4, datastream 161 is demultiplexed 162 into smaller stream blocks. IndividualCRCs 163 are attached to these smaller stream blocks and then separatecoding 164 and modulation 165 is performed on these smaller blocks.These smaller blocks are then transmitted via separate MIMO antennas166. It should be noted that in case of multi-code word MIMOtransmissions, different modulation and coding can be used on each ofthe individual streams resulting in a so called PARC (per antenna ratecontrol) scheme. Also, multi-code word transmission allows for moreefficient post-decoding and interference cancellation because a CRCcheck can be performed on each of the code words before the code word iscancelled from the overall signal. In this way, only correctly receivedcode words are cancelled to avoid any interference propagation in thecancellation process.

When the transmission channels between the transmitters and thereceivers are relatively constant, it is possible to use a closed-loopMIMO scheme to further improve system performance. In a closed-loop MIMOsystems, the receiver informs the transmitter of the feedbackinformation regarding the channel condition. The transmitter utilizesthis feedback information, together with other considerations such asscheduling priority, data and resource availability, to optimize thetransmission scheme.

A popular closed-loop MIMO scheme is MIMO precoding. With precoding, thedata streams to be transmitted are precoded, i.e., pre-multiplied by aprecoding matrix, before being passed on to the multiple transmitantennas in a transmitter.

An optional precoding protocol that employs a unitary pre-coding beforemapping the data streams to physical antennas is shown in FIGS. 5A and5B. The optional precoding creates a set of virtual antennas (VA) 171before the pre-coding. In this case, each of the codewords ispotentially transmitted through all the physical transmission antennas172. Two examples of unitary precoding matrices, P₁ and P₂ for the caseof two transmission antennas 172 may be:

$\begin{matrix}{{P_{1} = {\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}}},{P_{2} = {\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}}}} & (1)\end{matrix}$

Assuming modulation symbols S₁ and S₂ are transmitted at a given timethrough stream 1 and stream 2 respectively. Then the modulation symbolT₁ after precoding with matrix P₁ in the example as shown in FIG. 5A andthe modulation symbol T₂ after precoding with matrix P₂ in the exampleas shown in FIG. SB can be respectively written as:

$\begin{matrix}{{T_{1} = {{P_{1}\begin{bmatrix}S_{1} \\S_{2}\end{bmatrix}} = {{{\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}} \times \begin{bmatrix}S_{1} \\S_{2}\end{bmatrix}} = {\frac{1}{\sqrt{2}}\begin{bmatrix}{S_{1} + S_{2}} \\{S_{1} - S_{2}}\end{bmatrix}}}}}{T_{2} = {{P_{2}\begin{bmatrix}S_{1} \\S_{2}\end{bmatrix}} = {{{\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}} \times \begin{bmatrix}S_{1} \\S_{2}\end{bmatrix}} = {\frac{1}{\sqrt{2}}\begin{bmatrix}{S_{1} + S_{2}} \\{{j\; S_{1}} - {j\; S_{2}}}\end{bmatrix}}}}}} & (2)\end{matrix}$

Therefore, the symbols

$T_{11} = \frac{( {S_{1} + S_{2}} )}{\sqrt{2}}$ and$T_{12} = \frac{( {S_{1} - S_{2}} )}{\sqrt{2}}$

will be transmitted via antenna 1 and antenna 2, respectively, whenprecoding is done using precoding matrix P₁ as shown in FIG. 5A.Similarly, the symbols

$T_{21} = \frac{( {S_{1} + S_{2}} )}{\sqrt{2}}$ and$T_{22} = \frac{( {{j\; S_{1}} - {j\; S_{2}}} )}{\sqrt{2}}$

will be transmitted via antenna 1 and antenna 2, respectively, whenprecoding is done using precoding matrix P₂ as shown in FIG. 5B. Itshould be noted that precoding is done on an OFDM subcarrier levelbefore the IFFT operation as illustrated in FIGS. 5A and 5B.

An example of MIMO precoding is Fourier-based precoding. A Fouriermatrix is a N×N square matrix with entries given by:

P _(mn) =e ^(j2πmn/N) m,n=0,1, . . . (N−1)  (3)

A 2×2 Fourier matrix can be expressed as:

$\begin{matrix}{P_{2} = {{\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\1 & ^{j\pi}\end{bmatrix}} = {\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}}}} & (4)\end{matrix}$

Similarly, a 4×4 Fourier matrix can be expressed as:

$\begin{matrix}{P_{4} = {{\frac{1}{\sqrt{4}}\begin{bmatrix}1 & 1 & 1 & 1 \\1 & ^{{j\pi}\text{/}2} & ^{j\pi} & ^{{j3\pi}\text{/}2} \\1 & ^{j\pi} & ^{j2\pi} & ^{j3\pi} \\1 & ^{{j3\pi}\text{/}3} & ^{j3\pi} & ^{{j9\pi}\text{/}2}\end{bmatrix}} = {\frac{1}{\sqrt{4}}\begin{bmatrix}1 & 1 & 1 & 1 \\1 & j & {- 1} & {- j} \\1 & {- 1} & 1 & {- 1} \\1 & {- j} & {- 1} & j\end{bmatrix}}}} & (5)\end{matrix}$

Multiple precoder matrices can be defined by introducing a shiftparameter (g/G) in the Fourier matrix as given by:

$\begin{matrix}{{P_{mn} = ^{\frac{{j2\pi}\; m}{N}{({n + \frac{g}{G}})}}}{m,{n = 0},{{1\mspace{11mu}.\ldots}\mspace{11mu} ( {N - 1} )}}} & (6)\end{matrix}$

A set of four 2×2 Fourier matrices can be defined by taking G=4. Thesefour 2×2 matrices with g=0, 1, 2 and 3 are written as:

$\begin{matrix}{{P_{2}^{0} = {\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}}}{P_{2}^{1} = {\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\^{{j\pi}\text{/}4} & {- ^{{j\pi}\text{/}4}}\end{bmatrix}}}} & (7) \\{{P_{2}^{2} = {\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\^{{j\pi}\text{/}2} & ^{{j3\pi}\text{/}4}\end{bmatrix}}}{P_{2}^{3} = {\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\^{{j3\pi}\text{/}4} & {- ^{{j3\pi}\text{/}4}}\end{bmatrix}}}} & (8)\end{matrix}$

In a transmission path from a base station to a user equipment (UE),i.e., downlink transmission, the precoding matrix is usually determinedin dependence upon a precoding feedback information that is transmittedby the user equipment to the base station. The precoding feedbackinformation typically includes precoding-matrix identity.

When the total bandwidth in an OFDM system is divided into a pluralityof subbands, each subband being a set of consecutive subcarriers, due tofrequency-selective fading in the OFDM system, the optimal precoding fordifferent subbands (SBs), can be different, as shown in one exampleillustrated in FIG. 6. That is, in FIG. 6, different SBs use differentprecoding matrix. Subband 1 (SB1) which includes continuous OFDMsubcarriers 1 through 64, use precoding matrix F₂ ²; SB2 which includescontinuous OFDM subcarriers 65 through 128, use precoding matrix P₂ ¹,etc. Therefore, the precoding feedback information is transmitted on asubband basis. Moreover, due to feedback errors, the base station alsoneeds to inform the user equipment of the precoding information used ontransmitted subbands. This results in additional signaling overhead inthe downlink.

Besides precoding information, another form of feedback information isrank information, i.e., the number of MIMO layers. A MIMO layer is aspatial channel that can carry data symbols. It is well known that evenwhen a system can support 4×4 MIMO, rank-4 (4 MIMO layers) transmissionsare not always desirable. The MIMO channel experienced by the UEgenerally limits the maximum rank that can be used for transmission. Ingeneral for weak users in the system, a lower rank transmission ispreferred over a higher rank transmission from the throughputperspective. Moreover, due to frequency-selective fading, optimal rankmay be different on different subbands. As shown in the example of FIG.7, SB1 uses rank-1 transmission; SB2 uses rank-2 transmission, etc.Therefore, the UE needs to include the rank information in the feedbackinformation on a subband basis. Also, due to a possibility of feedbackerrors, the base station additionally needs to indicate the transmittedMIMO rank on different subbands. The rank information can also be commonacross the subbands, that is, a single rank value is reported for allthe subbands. In any case, this results in additional overhead on thedownlink.

Still another form of MIMO feedback information is layer orderinginformation. In the example of FIG. 8, the layer order for SB1, SB2,SB4, SB5 and SB8 is layer 2, and then layer 1; while the layer order forSB3, SB6 and SB7 is layer 1, and then layer 2. The layer orderinginformation is generally transmitted by the UE and also indicated by thebase station in control signaling on the downlink. The ordering oflayers can be based on the channel quality they experience or othersimilar criteria.

Another form of MIMO feedback information which applies to both MIMO andnon-MIMO scenarios is the selected subbands for transmission. In thiscase, the MIMO feedback information such as precoding, rank, IDs ofselected layers and layer ordering is provided for the selected subbandsonly. In this case, however, both the UE and the base station need tosignal the information on the selected subbands.

In packet-based wireless data communication systems, a control signalaccompanies the data transmission as shown in FIG. 9. In the 3^(rd)Generation Long Term Evolution (3G LTE) system, the control channel thatcarries the control signal is referred to as Physical Downlink ControlChannel (PDCCH). The PDCCH carries information such as UE ID, resourceassignment information, Payload size, modulation, Hybrid AutomaticRepeat-reQuest (ARQ) HARQ information, MIMO related information.

As described above, when the base station transmits data to the userequipment, the base station determines a transmission format independence upon the MIMO information that is inform by the userequipment through a feedback message. Contemporarily, the base stationtransmits the MIMO information, based on which the transmission formatof the data is determined, together with the data, to the userequipment.

In the present invention, we have constructed a protocol where the basestation does not need to explicitly signal those items of the MIMOinformation such as precoding, rank, selected MIMO layers and layerordering, etc. in a downlink transmission. The base station simplyindicates the identification of the feedback message to the userequipment in conformance with the protocol used by the base station toperform the MIMO transmission format.

FIG. 10 illustrates MIMO feedback and signaling according to a firstembodiment of the principles of the present invention. In this firstembodiment, a base station (BS) 210 simply indicates the identificationof the feedback message to UE 220 in accordance with the protocol usedby base station 210 to perform the MIMO transmission format.Specifically, at time t, US 220 transmits feedback message A 221 inresponse to a reference signal or a pilot signal received from basestation 210. Feedback (FB) message A 221 contains information such asselected subbands, precoding, rank and layer ordering, etc. At the sametime, when UE 220 transmits feedback message A 221, UE 220 also storesthe information in feedback message A 221 in a buffer (not shown). Attime (t+1), base station 210 sends control message 222 and data 223 toUE 220. Instead of transmitting feedback message A back to UE 220, basestation 210 indicates in the control message that the format for datatransmission is determined based on feedback message A 221.Subsequently, UE 220 already knows the feedback information feedbackmessage A and therefore reads the feedback message A 221 stored in thebuffer and processes the received data transmission according to theinformation in feedback message A. In this way, base station 210 doesnot have to explicitly transmit the precoding or other MIMO informationsuch as rank and layer ordering to UE 220 in the downlink transmission.At time (t+2), base station 210 receives updated feedback informationcarried in feedback message B 224 from UE 220. Feedback message B 224 isgenerated in response to another reference signal received from basestation 210. At time (t+3), base station 210 performs data transmissionto UE 220 using the updated feedback information feedback message B 224.Base station 210 also indicates in control message 225 that thecondition for data transmission including precoding, rank and layerordering is determined based on feedback message B. UE 220 thenprocesses received information data 226 according to the informationindication in feedback message B 224 that UE 220 has already stored.

In a second embodiment according to the principles of the presentinvention as shown in FIG. 11, base station 210 uses feedback from anearlier feedback message because the most recent feedback messagecontains error as detected by some erasure detector or Cyclic RedundancyCheck (CRC) unit. In the example of FIG. 11, base station 210 performs atransmission at time (t+3) according to feedback message A 221 receivedearlier at time t because feedback message B 227 received at time (t+2)contained errors. UE 210 then processes the received data 229 accordingto the information, such as precoding, rank and layer orderinginformation, carried in feedback message A 221. This scheme assures thatthe UE always decodes the information using a correct format asconfirmed by the base station.

In a third embodiment of the principles of the present invention asshown in FIG. 12, the Feedback (FB) messages are numbered with sequencenumbers 0, 1, 2 and 3 in the order of generation by UE 220. This wouldrequire 2-bits overhead to indicate the sequence numbers 0 through 3.Base station (BS) 210 informs UE 220 of the sequence number of the FBmessage that is used for determining format for MIMO transmission. Inthe third embodiment as shown in FIG. 12, the time for data transmissionis divided into a plurality of subframes. In subframe #1, BS 210receives FB #0 message. In subframe #2, BS 210 performs datatransmission to UE 220 according to the transmission format determinedbased the information carried in FB#0, and simultaneously receives FB#1message from UE 220. In subframe#3, the transmission is performed by BS210 according to the transmission format determined based on FB#1message received in subframe#2, and BS 210 simultaneously receives FB#2message from UE 220. But BS 210 detects that FB#2 message containserror. Therefore, base station 210 ignores FB#2 message and performstransmission according to FB#1 message in subframe#4. This schemeassures that UE always know the FB message used to determine the format(i.e. precoding, rank and layer ordering etc.) of the MIMO transmission.

In a fourth embodiment according to the principles of the presentinvention as shown in FIG. 13, the Feedback (FB) messages transmitted byUE 220 are not sequentially numbered. Instead, base station 210transmits one of four possible combinations of binary symbols 0 and 1.These combinations indicate which previously received FB message is usedto determine format for MIMO transmission. Based on the receivedcombination, UE 220 can determine which FB message is actually used forMIMO transmission in a given subframe. In the example of FIG. 13, insubframe#2 and subframe#3, base station 210 transmits FB(0) message(i.e., combination of ‘0’ and ‘0’) in control signal, which indicatesthat the most recent FB message was used for determining the MIMOtransmission format. In subframe#3, FB message is received in error, andtherefore in subframe#4, base station 210 indicate FB(1) message (i.e.,combination of ‘0’ and ‘1’) which means that the FB message receivedprior to the most recent message is used for determining MIMOtransmission format. UE 220 can then processes the received signalaccording to the FB message which is sent in subframe#2 because UE 220knows that FB message in subframe#3 was received in error. Insubframe#5, the received FB message contains error, and therefore insubframe#6 base station 210 transmits FB(1) message to UE 220 toindicate that the transmission format in subframe#6 was determined basedon the FB message that is earlier than the most recent FB transmitted byUE 220. In subframe#6, the received FB message contains error again, andtherefore in subframe#7 base station 210 transmits FB(2) message (i.e.,combination of ‘1’ and ‘0’) to UE 220 to indicate that the transmissionin subframe#7 was determined based on the FB message received before thetwo recent FB messages received from the UE. Note that the formats fortransmission by BS 210 in subframe#6 and subframe#7 are the same becausethere was no correctly received FB message in subframe#6 and subframe#7.It should also be noted that the same goal can be achieved if thecombinations indicate how many previous consecutive FB messages werereceived in errors with indication of 0 through 3.

In a fifth embodiment according to the principles of the presentinvention as shown in FIG. 14, the Feedback (FB) messages are notsequentially numbered. We also assume that MIMO FB messages are sent inevery 2 subframes. Note that the MIMO feedback rate in the time domaincan be configured by the network. In the fifth embodiment, base station210 indicates the subframe number in which the FB is received and isused for determining MIMO transmission format in the downlink. Thisindication is done on the downlink control channel that accompanies thedownlink data transmission. As shown in FIG. 14, in subframe#2 andsubframe#3, the MIMO transmission format, which includes precoding,rank, IDs of selected layers and layer ordering, is determined using theFB message received in subframe#1. This is indicated by FB(1)indication. In subframe#4 through subframe#7, the MIMO transmissionformat is determined according to the FB message received in subframe#3.This is indicated by FB(3) indication. Note that FB message insubframe#5 is received in error and hence is not used. Finally insubframe#8, the MIMO format corresponding to FB message in subframe#7 isused.

In a sixth embodiment of the current invention, a 1-bit indication isused to indicate whether the MIMO transmission format includingprecoding, rank, IDs of selected layers and layer ordering informationis determined based on the most recent UE feedback message or not. Ifthe base station decides to use another transmission format than the onedetermined based on the most recent UE feedback message for datatransmission, this other transmission format is explicitly transmittedto the UE as shown in FIG. 15. A ‘0’ in the control informationindicates that the MIMO transmission format is determined using the mostrecent FB message received. A ‘1’ in the control information indicatesthat the MIMO transmission format is carried explicitly. The MIMOtransmission format can then be separately coded, modulated or jointlycoded, modulated with other downlink control information and transmittedby the base station. An explicit MIMO format indication may be necessarywhen the base station uses different MIMO format than one reported bythe UE or if the most recent FB message was received erroneously andbase station uses MIMO format according to an earlier FB message.

In a seventh embodiment according to the principles of the currentinvention as shown in FIG. 16, the MIMO feedback is provided for part ofthe whole bandwidth in a given FB message. In the example of FIG. 16,FB#1 covers the lower half of the bandwidth while FB#2 covers the upperhalf of the bandwidth. In this case, the base station can indicate thatMIMO transmission format (precoding, rank, IDs of selected layers andlayer ordering etc.) is determined either according to FB#1 if the UE isscheduled in the left half of the bandwidth or according to FB#2 if theUE is scheduled in the right half of the bandwidth. If the UE isscheduled in both left and right halves then the base station needs toindicate both FB#1 and FB#2. Also, the base station can simply indicatethe MIMO transmission format by 1-bit indication if the most recent FB#1and FB#2 are used for transmission or MIMO transmission format isexplicitly indicated.

In an eighth embodiment according to the principles of the currentinvention as shown in FIG. 17, the base station always transmit the MIMOtransmission format explicitly, and the user equipment always try todecode the control message that contains the MIMO transmission formatinformation. FIG. 17 illustrates a flow chart of the processing of MIMOsignals according to the principles of the current invention. In thisembodiment, no 1-bit indication informing the UE if MIMO format isexplicitly signaled or not is used in the regular control message. Aftera UE receives control message through control channels from a basestation (step S10), the UE decodes the control message carried throughthe control channels at step S20 to determine which part of the controlmessage carries MIMO transmission format information and which part ofthe control message carries information regarding the UE ID/CRC, etc.Then at step S30, the UE determines whether the decoding processperformed at step S20 is successful. If the decoding process is notsuccessful, the UE stops the process (step S50). Otherwise, if thedecoding process at step S20 is successful, the UE tries to decode thepart of the control message that contains the MIMO transmission format(precoding, rank and selected layers etc.) at step S40. Then at stepS60, the UE determines whether the decoding process performed at stepS40 is successful. If the decoding succeeds, the UE processes the signalreceived from the base station according to MIMO transmission formatinformation contained in the control message (step S80). If the decodingfails, the UE processes the signal received from the base stationassuming that the base station used the MIMO transmission format that isdetermined based on to most recent Feedback message transmitted from theUE.

In a ninth embodiment according to the principles of the currentinvention, the base station uses a plurality of common MIMO transmissionformats (precoding, rank and selected layers etc.) on all the datascheduled to be transmitted to the UE in cases when the base stationdoes not use the MIMO transmission format that is determined based onthe FB message reported by the UE. In this way, a MIMO format indicationfield including a fixed amount of bits (2-6 bits) can be included in thePhysical Downlink Control Channel (PDCCH) as shown in Format I in FIG.18. In case where the BS uses the subband specific MIMO transmissionformat determined based on the UE feedback, a certain combination ofbits (e.g. ‘00’ for 2-bits and ‘000000’ for 6-bits) included in the MIMOformat field will indicate this. As shown in Table 1, ‘000000’ in theMIMO format indication field in PDCCH indicates that the base stationuses the MIMO transmission format based on the most recent FB messagetransmitted from the UE; ‘000001’ in the MIMO format indication field inPDCCH indicates that the base station uses the MIMO transmission formatbased on one-earlier than the most recent FB message transmitted fromthe UE; ‘000010’ 000001′ in the MIMO format indication field in PDCCHindicates that the base station uses the MIMO transmission format basedon two-earlier than the most recent FB message transmitted from the UE.Therefore, if the base station is using subband specific MIMOtransmission format determined based on UE feedback, the MIMO formatfield in the PDCCH will indicate this. In case when the base stationoverrides the UE feedback, the base station transmits a MIMO formatindication selected from ‘000011’ through ‘111111’ to the UE to indicatethat a common MIMO format selected from the sixty-one common MIMOtransmission formats will be used for all the allocated resource blocks(RB, i.e., the minimum frequency subband) and indicated in the MIMOformat field. It should be noted that in this case another format Format0 (not including MIMO format field) may be required for PDCCH. Thisformat can be used in cells not supporting MIMO. It should be noted thatthere will be no need to perform a blind detection between Format 0 andFormat I because these formats will not be used simultaneously in acell.

TABLE 1 An Example of MIMO format indication on the downlink for supportof SU-MIMO 6-bits MIMO Information (MI) in the downlink Purpose ‘000000’BS uses MIMO transmission format determined based on the most recentFeedback from the UE ‘000001’ BS uses MIMO transmission formatdetermined based on one-earlier than the most recent UE Feedback fromthe UE (assuming BS detected errors in the most recent UE Feedback)‘000010’ BS uses MIMO transmission format according to two- earlierFeedback than the most recent UE Feedback from the UE UE (assuming BSdetected two consecutive errors in the UE Feedback) ‘000011- BSoverrides the UE feedback and uses a common MIMO 111111-’ transmissionformat for all the RBs allocated to the UE. This common MIMOtransmission format is indicated by one out of the 61 combinations(4-63).

It should be noted that in the ninth embodiment of the presentinvention, the UE needs to store the information carried in the threeprevious Feedback messages. Based upon the MIMO Information (MI) fieldin the PDCCH, the UE uses the information in the corresponding Feedbackmessage or the MIMO format indicated in the MIMO format field to processthe received MIMO signal. It should be noted that this approach alsoapplies to the schemes where UE Feedback is provided for the “best-M”subbands (the subbands having the highest channel quality indication) orover part of the total bandwidth.

It is also possible to use another bits combination in another field inthe control message such as payload size field, modulation, HARQinformation or resource allocation fields indicating that Node-B isfollowing the MIMO format according to the feedback from the UE.

1-23. (canceled)
 24. A method for receiving data at a user equipment(UE), comprising: generating a feedback message in response to each of aplurality of reference signals received from a base station (BS), thefeedback message comprising at least one rank indicator; transmittingeach feedback message to the BS; and receiving a data signal and aPhysical Downlink Control Channel (PDCCH) from the BS, the data signaltransmitted according to the at least one rank indicator.
 25. The methodof claim 24, wherein the at least one rank indicator comprises aplurality of rank indicators, each rank indicator configured to indicatea rank transmission for a corresponding frequency subband.
 26. Themethod of claim 25, wherein the data signal is transmitted to the UE ona plurality of frequency subbands according to the rank indicatorcorresponding to each of the frequency subbands.
 27. The method of claim25, further comprising selecting an optimal rank for each of thefrequency subbands based on frequency-selective fading.
 28. The methodof claim 24, wherein the at least one rank indicator comprises a singlerank indicator for all frequency subbands.
 29. The method of claim 24,wherein the PDCCH comprises an indication of a transmitted rank for eachfrequency subband from the BS.
 30. The method of claim 29, wherein amultiple-input multiple-output (MIMO) transmission format indicationfield in the PDCCH comprises the indication of the transmitted rank foreach frequency subband.
 31. A user equipment (UE) configured to receivedata, the UE comprising: a feedback message generator configured togenerate a feedback message in response to each of a plurality ofreference signals received from a base station (BS), the feedbackmessage comprising at least one rank indicator; a transmitter configuredto transmit each feedback message to the BS; and a receiver configuredto receive a data signal and a Physical Downlink Control Channel (PDCCH)from the BS, the data signal transmitted according to the at least onerank indicator.
 32. The UE of claim 31, wherein the at least one rankindicator comprises a plurality of rank indicators, each rank indicatorconfigured to indicate a rank transmission for a corresponding frequencysubband.
 33. The UE of claim 32, wherein the data signal is transmittedto the UE on a plurality of frequency subbands according to the rankindicator corresponding to each of the subbands.
 34. The UE of claim 32,further comprising selecting an optimal rank for each of the frequencysubbands based on frequency-selective fading.
 35. The UE of claim 31,wherein the at least one rank indicator comprises a single rankindicator for all frequency subbands.
 36. The UE of claim 31, whereinthe PDCCH comprises an indication of a transmitted rank for eachfrequency subband from the BS.
 37. The UE of claim 36, wherein amultiple-input multiple-output (MIMO) transmission format indicationfield in the PDCCH comprises the indication of the transmitted rank foreach frequency subband.
 38. A base station, comprising: a transmitterconfigured to transmit a plurality of reference signals to a userequipment (UE); and a receiver configured to receive a plurality offeedback messages from the UE, each feedback message comprising at leastone rank indicator, wherein the transmitter is further configured totransmit a Physical Downlink Control Channel (PDCCH) to the UE andtransmit a data signal to the UE according to the at least one rankindicator.
 39. The base station of claim 38, wherein the at least onerank indicator comprises a plurality of rank indicators, each rankindicator configured to indicate a rank transmission for a correspondingfrequency subband.
 40. The base station of claim 39, wherein thetransmitter transmits the data signal to the UE on a plurality offrequency subbands according to the rank indicator corresponding to eachof the frequency subbands.
 41. The base station of claim 39, wherein therank indicator corresponding to each of the frequency subbands wasselected as an optimal rank for the each frequency subband based onfrequency-selective fading.
 42. The base station of claim 38, whereinthe at least one rank indicator comprises a single rank indicator forall frequency subbands.
 43. The base station of claim 38, wherein amultiple-input multiple-output (MIMO) transmission format indicationfield in the PDCCH comprises an indication of the transmitted rank foreach frequency subband.